A deep MeerKAT view of associated HI absorption in radio AGNs at intermediate redshift: Role of absorber geometry and conditions of the gas

TL;DR

This study uses MeerKAT to search for HI 21-cm absorption in 17 powerful radio AGNs at intermediate redshift ($0.25<z<0.7$), aiming to relate HI presence to AGN type, host ISM, absorber geometry, and gas conditions. Fifteen spectra yield three HI absorptions, including two associated absorptions in young CSS sources and one local intervening system, corresponding to a detection rate of $13\%\pm 7\%$; the absorptions show extremely low peak optical depths, consistent with high spin temperatures or partial coverage in bright radio environments. The results reinforce the trend that young, compact radio sources are more likely to exhibit HI absorption and demonstrate the critical role of absorber geometry and physical conditions in determining detectability, even at $z\sim0.3$–$0.7$. The findings underscore the value of high-resolution follow-up (VLBI) and multi-transition molecular data to map the full ISM distribution and temperatures, and have implications for interpreting upcoming blind HI surveys (e.g., FLASH) and future SKA capabilities.

Abstract

We present MeerKAT observations searching for HI absorption in a sample of 17 powerful ($L_{\rm 1.4GHz}> 10^{27}$ W Hz$^{-1}$) radio sources at intermediate redshifts ($0.25<z<0.7$). The sample is well characterised at radio and optical wavelengths, allowing us to connect the presence (or absence) of HI to the properties of the AGN and its host galaxy. The sample consists mostly of core-dominated sources and quasars. Half of the targets have a UV luminosity $L_{\rm UV} = 10^{23}$ W Hz$^{-1}$, above this limit, the gas would be expected to be ionised by this radiation. We obtained 15 spectra free (or almost free) of radio frequency interference, reaching extremely low optical depths ($τ_{\rm peak} < 0.005$) resulting in three new HI absorption detections. Two are associated HI absorptions, giving a detection rate of such systems of $13\%\pm 7\%$. Both are found in young radio sources (PKS 1151-34 and PKS 1306-09), confirming the trend that this type of sources are more often detected in HI compared to more evolved ones. The UV luminosity of both these sources is below $10^{23}$ W Hz$^{-1}$. Surprisingly, one of the detections (PKS 1151-34) is hosted by a quasar, suggesting that the radio lobes are still embedded in the circumnuclear disc. In the second source (PKS 1306-09), the HI is highly blueshifted and likely part of a jet-driven outflow. A third detection is a 'local intervening' system, caused by a galaxy in the local environment of PKS 0405-12 and located in front of the southern radio lobe of this source, about 100 kpc in projection from this quasar. Overall, the results indicate a variety of plausible situations, which resemble what is seen at low redshifts. For the associated absorption, a combination of evolutionary status of the radio sources, physical conditions, and geometry of the gas structure determine the detection rate of HI absorption.

Figures (12)

• Figure 1: Distribution of the radio luminosities for sources in various samples available in literature, adapted from Murthy22. Location of the present sample is indicated as the green area.
• Figure 2: Example of one of the continuum images obtained (PKS 1954--38), illustrating the image quality. The target, PKS 1954--38, is visible in the bottom of the image indicated with an arrow, and the residual artifacts around this source are due to the strength of the source. The total flux of PKS 1954--38 is 1.195 Jy and its peak is 940 mJy beam$^{-1}$. The root-mean-square level of the artefacts around PKS 1954--38 is 0.1 mJy beam$^{-1}$ while the noise level away from the target is 0.05 mJy beam$^{-1}$. This indicates that the dynamic range near the target is about $10^4$.
• Figure 3: Distribution of the peak optical depth taken from Maccagni17 with overlaid the peak optical depth for the detected (dark red boxes) and limits thereof (light red) for the sources of the present sample. The distribution shows how the present observations reach very low optical depths.
• Figure 4: Left panel: HI absorption profile in PKS 1151--34. Note: the RFI affecting part of the band. The error in the zero-point of the velocity scale is 30 $\,$km$\,$s$^{-1}$ (see Table \ref{['tab:obs']}). Right panel: VLBI image of PKS 1151--34 from Tzioumis02. The continuum emission spans $\sim$700 pc.
• Figure 5: Left panel: HI absorption profile against the young radio galaxy PKS 1306--09. The error in the zero-point of the velocity scale is 20 $\,$km$\,$s$^{-1}$ (see Table \ref{['tab:obs']}). Right panel: VLBI image of PKS 1306--09 taken from Tzioumis02. The size of the continuum emission is 2.4 kpc.